27 February 2018

DDH7/ DDH9/DDK9 ITA2 FSK and Sorcerer

Following the previous post, I tried the asynchronous FSK decoder of Sorcerer on some 5N1.5 bitstreams transmitted by DWD: precisely DDH7, DDH9 and DDK9. All transmissions are FSK 50Bd/450Hz with 5N1.5 framing, as evidenced by the SA rasters in Figure 1.

Fig. 1

I added the generic decoder for synchronous FSK demodulator in order to see and process the bitstreams on which Sorcerer works. As already underlined, bit editors work with an integer number of bits since they can't represent an half bit, so a 5N1.5 bitstream is depicted as 15-bit frame obtained by aggregating two consecutive frames (ie, 7.5 x 2). The raster method of SA allows to detect the length of 1.5 bit because it works in the time domain. Figure 2 indicates that in all three cases, Sorcerer correctly demodulates the signals.

Fig. 2

Unfortunately the asynchronous FSK decoder for ITA2 does not provide the 5N1.5 framing and therefore there are some uncertainties in the decoding; using 5N2 framing you get gibberish (Figure 3). It's worth noting that in the previous post a similar not-standard framing was correctly decoded by the S-4285 decoder using 5N1 either 5N2 settings.

Fig. 3

In these cases, asynchronous FSK with 1.5 framing, the best choice is Fldigi which provides this non-standard length of the stop bit (Figure 4).